Polyethylene oxide (PEO), polyacrylamide (PAAm), polyacrylic acid (PAAc) or polymethacrylic acid (PMAAc), which is a water-soluble polymer, dissolves in water in an aqueous solution within an optional temperature range. However, it has been described by Yoshihito Nagata and Tasuku Saito in Nippon Kagaku Kaishi (Journal of Japanese Chemical Society), 1, 171 (1976) that a mixed aqueous solution of PEO with PAAc or PMAAc forms an inter-polymer complex under an acidic condition, and the complex shows lower critical solution temperature (LCST) in the aqueous solution.
On the other hand, it has been described by D. J. Eustance et al. in J. Appl. Polym. Sci., 35, 707 (1988) that a mixed aqueous solution of PAAm with PAAc forms an inter-polymer complex under an acidic condition, and the complex shows upper critical solution temperature (UCST).
However, the polymers shown in the above form inter-polymer complexes with polyacrylic acid (PAAc) or polymethacrylic acid (PMAAc), which are polyelectrolytes, but the inter-polymer complexes are not formed under neutral to alkaline conditions due to dissociation of carboxyl groups of the polyelectrolytes, so that the application range has a limitation.
Accordingly, an object of the invention is to find a technique by which inter-polymer complexes can be formed by being responsive to temperature even under neutral to alkaline conditions.